The site has been growing fast for several years now
Tumblr has been growing fast over the last few years to the point that it's now more popular than blogs, at least in Google searches. The term "tumblr" was used in more Google searches than the term "blog" for the first time last year.This is due to a decline in searches for blog and a huge increase in searches for tumblr.
The growth in searches for blog slowed down after 2007 and peaked sometime in 2009. It's been on the decline ever since, reaching levels last seen in January 2006.
Searches for Tumblr only started picking up steam in January 2010, but exploded after that. It doesn't look like the interest is slowing down and Tumblr's own usage numbers also show that more and more people set up blogs on Tumblr or simply visit the site.